1. Field of the Invention
The present invention relates to a motor drive control device for a so-called electric power-assisted vehicle such as a bicycle with a motor.
2. Description of Related Art
Power transmission systems of an electric power-assisted bicycle include several variations shown in FIGS. 1 to 5. For these configurations, a one-way clutch is installed on the rear wheel gear (hereinafter referred to as an R gear). A decelerator may or may not be provided depending on the torque and speed characteristics of the motor.
FIG. 1 shows a first configuration in which a transmission system that transmits torque from a motor to a rear wheel and a transmission system that transmits torque from a pedal to the rear wheel share a gear shifter. In the first configuration, both the pedal and the motor drive the same front gear (hereinafter referred to as the F gear), and the front wheel is not driven.
FIG. 2 shows a second configuration in which a transmission system that transmits torque from a motor to a rear wheel and a transmission system that transmits torque from a pedal to the rear wheel share a gear shifter. In the second configuration, the middle gear of the chain driven by the pedal is also driven by the motor. The front wheel is not driven in this configuration either.
FIG. 3 shows a third configuration in which a transmission system that transmits torque from a motor to a rear wheel and a transmission system that transmits torque from a pedal to the rear wheel share a gear shifter. In the third configuration, the rear wheel is driven by pedal and motor via two chains, respectively. The front wheel is not driven in this configuration either.
FIG. 4 shows a first configuration in which a gear shifter is installed only in the drive route from the pedal. In this configuration, a rear wheel motor drives a real wheel hub (corresponding to the black circle in FIG. 4) to the rear of the gear shifter. In this configuration, driving is conducted with the rear wheel motor further towards the rear wheel than a one way clutch installed in the R gear, and thus, it is possible to use an electromagnetic brake. The front wheel is not driven in this configuration either.
FIG. 5 shows a second configuration in which a gear shifter is installed only in the drive route from the pedal. In this configuration, the motor drives the front wheel. Because there is no one way clutch on the front wheel side, it is possible to use an electromagnetic brake.
In the configurations shown in FIGS. 1 to 3, pedal input torque and assist motor torque from the motor both drive the rear wheel through the gear shifter, and thus, even if the gear shift position, or in other words the gear ratio, changes, the ratio of the torque applied by the pedal to drive the drive wheel (in this case the rear wheel) to the torque applied by the motor to drive the drive wheel, or in other words the assist ratio, does not change. However, because in all of these configurations, the pedal input torque and the assist motor torque work through the R gear, the torque is applied through the one way clutch provided in the R gear. Thus, while torque in the acceleration direction is transmitted to the rear wheel from the motor, torque from the electromagnetic brake, which is in the opposite direction, is not transmitted. In other words, with these configurations, it is not possible to use an electromagnetic brake that includes an electrical power regenerative brake.
On the other hand, in the configurations shown in FIGS. 4 and 5, motor torque is transmitted directly to the rear of the one way clutch installed in the R gear or directly to the front wheel, and thus, it is possible to use an electromagnetic brake that includes an electrical power regenerative brake. However, these configurations have disadvantages as described below.
In the following description, it is assumed that the gear shifter is a three-speed gear shifter, and that the H (high speed) position of the gear shifter has a gear ratio of 4/3, the M (mid speed) position has a gear ratio of 1, and the L (low speed) position has a gear ratio of 3/4.
Specifically, if the same amount of assist motor torque is applied for the same amount of pedal input torque regardless of gear ratio, in the H position, a pedal input torque of 3/4 (the inverse of the gear ratio 4/3) that of the M position is applied to the rear wheel. However, because the assist motor torque is applied directly to the front wheel or the rear wheel without being transmitted through the gear shifter, the amount of assist motor torque remains the same as when the M position is used. Thus, the assist ratio is 1/(3/4) or 4/3 that of when the M position is used. Conversely, in the L position, the assist ratio is 1/(4/3) or 3/4 that of when the M position is used.
Despite the fact that the L position is used during high load situations such as when accelerating from a standstill, climbing hills, or the like, the assist ratio decreases, and when the H position is used, typically during low load situations, the assist ratio is increased.
Also, in some cases, legal regulations or the like stipulate that the maximum assist ratio be a function of the speed of the vehicle. For example, according to Japanese law, there are restrictions on the average assist ratio (the average assist ratio during a ripple fluctuation cycle if the assist ratio has such a ripple fluctuation cycle) such as that shown in FIG. 6. In other words, the maximum average assist ratio needs to follow a curve in which the assist ratio is maintained at 2 up to 10 km/h, and between 10 km/h and 24 km/h inclusive, the assist ratio decreases in a linear manner such that the maximum average assist ratio at 24 km/h is 0.
Also, in such a case, whether the system meets the requirements of the regulations is determined based on the H position in which the assist ratio is greater, and thus, in the M position or the L position, it is not possible to maximize the assist ratio to the fullest within the legal framework.
In this example, the gap between the average assist ratios of the H position and the L position is (3/4)/(4/3)=9/16 times. Even if the maximum allowed assist ratio of 2 is used in the H position, the ratio is 9/8 (=2*9/16) in the L position, which means that the average assist ratio is only slightly higher than half of the maximum legal limit.
As schematically shown in FIG. 7, if the average assist ratio in the M position is 3/2, the average assist ratio in the H position is 2, and the average assist ratio in the L position is 9/8. On the other hand, if the average assist ratio in the M position is 1, then the average assist ratio in the H position is 4/3 and the average assist ratio in the L position is 3/4.
Thus, even though a greater assist motor torque is desired when the user sets the gear in the L position, the assist ratio becomes smaller, which means that a problem arises in that the drive power of the motor cannot be effectively used.
A technique has been disclosed in which the ratio of the electric motor to human driving power is small at a high speed setting and large at a low speed setting in order to encourage the rider to shift to a gear appropriate to the running speed, with the view that there is a problem that the motor efficiency is low when a vehicle with a power transmission system of a configuration similar to that shown in FIG. 1 runs at low speeds even in high gear. According to this technique, if the vehicle runs at low speeds in high gear, the assist ratio by the motor becomes small, thus increasing the amount of human driving power required. In other words, the rider is encouraged to shift gears by the discomfort of having to provide more driving power, and when the gear is actually shifted, a large assist ratio is provided in low gear. In that disclosure, the power transmission system of the configuration shown in FIG. 1 is used, and use of such a ratio setting for the power transmission system of the configurations shown in FIGS. 4 and 5 has not been considered.